I think there's a couple of issues with this specifically with regards to IIDX (or any music game) at D&B:That's amazing. IIDX would look great at D&B, too. No idea if it'd appeal to their demographic,but Bemani faithful would flock there in droves.
1) The average DDR/IIDX (and possibly SDVX*?) player is nowhere in the universe of D&B's usual clientele. It would be a leap of faith for them to think the games would do well at every location, even if it does OK at a specific location (like in LA or NY or something).
2) The coin drop rates on a DDR or IIDX machine will never rate above average at their locations. An average game of either game takes about 8-10 minutes, whereas plays on most redemption games average under a minute (if that). They would need to be consistently in use to be worth it -- which is not impossible, but again, a stretch.
3) Maintenance. Notice that this is a question that the VP doing the AMA keeps dodging -- their locations have traditionally been awful regarding machine upkeep, which is something that is pretty much required for a music game to do well. If the machine doesn't respond well, core players are not going to waste their time -- and for these games, that is the market. They're not going to bother playing DDR on a pad that drops 1 in 100 inputs, or SDVX on a cabinet that misfires lasers one or more times per stage.
*This comes from the AMA as well, specifically called out as a music game they think they could carry due to easier licensing logistics. I don't know how true that is, but it didn't seem like a huge lift for Round 1 so it may not be that far off?